The total cost can be model by the equation:

where y is the total cost, x is the number of service calls, m is the cost for each service call and b is the fixed cost.
Plugging the values given in the problem we have that the equation is:

if we have 135 service calls that means that x=135; plugging this value in the equation fo the cost we have:

Therefore the cost in one month with 135 service calls is $8050